(1) Field of the Invention
This invention relates to a semiconductor light-emitting element, particularly usable for a white light-emitting diode.
(2) Related Art Statement
Recently, various light-emitting diodes (LEDs) are widely available. LEDs are expected for illuminating use as well as displaying use because of their low electric power consumption, long life time, CO2 gas reduction originated from the reduction of the high energy consumption such as the low electric power consumption, and thus, much demand for the LEDs are expected.
As of now, the LEDs are made of various semi-conducting material such as GaAs-based semi-conducting material, AlGaAs-based semi-conducting material, GaP-based semi-conducting material, GaAsP-based semi-conducting material and InGaAlP-based semi-conducting material, and thus, can emit various color lights from red to yellow-green. Therefore, the LEDs are employed particularly for various displaying use. Recently, blue and green LEDs have been realized by using GaN-based semi-conducting material. As a result, selecting a given LED, a given color light from red to blue, that is, within visible light range, can be obtained from the LED, and full-color displaying is also realized. Moreover, white light-emitting diodes (white LEDs) are being realized by using RGB LED chips or using two color lights-emitting diodes composed of blue LEDs with yellow fluorescent substance thereon. As a result, LED illumination is being realized at present.
However, the white LED using the RGB LED chips requires higher cost because the plural LED chips are employed, so that in view of the cost, it is difficult to employ the white LED for illumination use. On the other hand, full color can not be recognized by the white LED using the two color lights-emitting diode because it employs only two primary colors, not three primary colors. Moreover, in the white LED, the brightness of only about 25 lm/w can be realized, which is very small as compared with the brightness of 90 lm/W of a fluorescent tube.
Therefore, a white LED employing three primary colors is strongly desired all over the world because of the low energy consumption taking environmental problem into consideration. In reality, such a white LED is intensely developed by Japanese national professions and foreign major electric-manufacturing enterprises.
Such an attempt is made as to fabricate a white LED using three or over primary colors as illuminating a three primary colors-fluorescent substance by an ultraviolet LED. This attempt is fundamentally based on the same principle as a fluorescent tube, and employs the ultraviolet LED as the ultraviolet beam from the mercury discharge in the fluorescent tube. In this case, the cost of the white LED is increased because the three primary colors-fluorescent substance is additionally employed for the ultraviolet LED. Using a GaN-based semi-conducting material, a blue LED can be realized, and then, using the GaN-based semi-conducting material, the ultraviolet LED can be realized. However, the luminous efficiency of the resulting ultraviolet LED is largely reduced, as compared with the blue LED.
The luminescence reduction is considered as follows. If the GaN-based semiconductor film is epitaxially grown on a substrate made of e.g., a sapphire single crystal, much misfit dislocations are created at the boundary between the film and the substrate due to the difference in lattice constant between the film and the substrate. The misfit dislocations are propagated in the film and a light-emitting layer provided on the film, and thus, many dislocations are created in the resulting LED.
In a blue LED or a green LED made of GaN-based semi-conducting materials, the light emitting layer is made of an InGaN semi-conducting material. In this case, the In elements are partially located, and thus, some carriers are located and confined. Therefore, the carriers are recombined before they are moved and seized at the dislocations, so that the LED can exhibit its sufficient luminous efficiency.
That is, even though much dislocation are created in the light-emitting layer, the carriers are recombined and thus, a given luminescence is generated before they are moved and seized at the dislocation as non-luminescence centers, so that the blue LED or the green LED using the GaN-based semi-conducting materials can exhibit their high luminous efficiency.
For fabricating an ultraviolet LED, the In ratio of the light-emitting layer must be reduced. Therefore, the In elements are not almost located, and thus, the diffusion length of carrier is elongated. As a result, the carriers are easily moved at and recombined with the dislocations in the light-emitting layer. In this way, the luminous efficiency of the ultraviolet LED is reduced due to the large amount of dislocation in the light-emitting layer, as compared with the blue LED. In this point of view, various dislocation-reducing method are researched and developed.
For example, such an ELO technique is proposed as fabricating a strip mask made of SiO2 during an epitaxial process and preventing the propagation of the misfit dislocations created at the boundary between the epitaxial film and a substrate. According to the ELO technique, a light-emitting layer having fewer dislocations can be formed above the substrate via the strip mask. However, the ELO technique is a complicated means, so that the manufacturing cost is increased. Then, in the ELO technique, a thicker layer made of e.g., a GaN-based semi-conducting material is formed on the substrate, which results in being curved. Practically, in a device manufacturing process, when epitaxial films are formed on their respective substrates by the ELO technique, the better half of the substrates is broken. Therefore, it is difficult to employ the ELO technique in a practical device manufacturing process, particularly for LEDs.
In addition, an attempt is made to epitaxially grow a bulky GaN single crystal for reducing the dislocation density of the resulting device, for example by using a high pressure solution growth method, a vapor phase epitaxial growth method or a flux method. As of now, however, such a bulky single crystal enough to be applied for the device manufacturing process is not grown and prospected.
For fabricating a bulky GaN single crystal of low dislocation density, an attempt is made to grow a thicker GaN single crystal on a substrate made of an oxide to match in lattice the GaN single crystal by a HVPE method, and thereafter, remove the substrate, to obtain only the GaN single crystal to be used as a substrate. However, the GaN single crystal enough to be industrially applied for LEDs has not been fabricated yet.
As a result, the high luminous efficiency in such a white LED as employing three or over primary colors through the illumination of a fluorescent substance by an ultraviolet LED is not technically prospected.
It is an object of the present invention to provide a new semiconductor light-emitting element preferably usable for a LED to emit an any color light regardless of the dislocation density, particularly a white LED.
For achieving the above object, this invention relates to a semiconductor light-emitting element including a substrate, an underlayer, formed on the substrate, made of a first semi-conducting nitride material including Al element and having a full width at half maximum (FWHM) in X-ray rocking curve of 90 seconds or below, a first conductive layer, formed on the underlayer, made of a second semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first cladding layer, formed on the first conductive layer, made of a third semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a light-emitting layer composed of a base layer, formed on the first cladding layer, made of a fourth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and plural isolated island-shaped single crystal portions, embedded in the base layer, made of a fifth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and having an in-plane lattice constant larger than that of the third semi-conducting nitride material, a second cladding layer, formed on the light-emitting layer, made of a sixth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, and a second conductive layer, formed on the second cladding layer, made of a seventh semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In. Then, the bandgap of the third semi-conducting nitride material constituting the first cladding layer, the bandgap of the fourth semi-conducting nitride material constituting the base layer and the bandgap of the fifth semi-conducting nitride material become larger by turns. That is, the relation of the handgap of the third semi-conducting nitride material greater than the handgap of the fourth semi-conducting nitride material greater than the handgap of the fifth semi-conducting nitride material is satisfied.
In this case, if the function of a conductive layer is given to the first cladding layer, the first conductive layer may be removed. Similarly, if the function of a conductive layer is given to the second cladding layer, the second conductive layer may be removed.
Recently, such a LED as to be illuminated through a light-emitting layer having mismatched in lattice and isolated island-shaped single crystal portions have been intensely researched and developed. In the LED, if the sizes of the single crystal portions are dispersed, various color lights are emitted from the single crystal portions. The emitted various color lights are superimposed, thereby to emit an any color light or a white color light from the light-emitting layer.
However, more dislocations are propagated and created in the isolated island-shaped single crystal portions due to the low crystallinity of an underlayer for the light-emitting layer, regardless of the sizes the single crystal portions. Therefore, the luminous efficiency of the light-emitting layer is degraded due to the low crystallinity thereof. As a result, such an any color LED or a white LED has not been realized yet.
Then, the inventors had been intensely studied to improve the crystallinity of the island-shaped single crystal portions constituting the light-emitting layer, and thus, made an attempt to improve the crystallinity of the underlayer by which the crystallinity of the single crystal portions are affected.
Conventionally, the function as a buffer layer is regarded as most important for the underlayer in a semiconductor light-emitting element, and as a result, attention is not paid to the crystallinity of the underlayer. In addition, it is desired for functioning as the buffer layer sufficiently that the underlayer has a relatively low crystallinity.
On the other hand, the inventor found out that, in the case of making a light-emitting element of Al-including semi-conducting nitride materials, if the crystallinity of the underlayer is developed to some degree, the underlayer can function as a buffer layer.
Therefore, if the crystallinity of the underlayer including Al element is developed to full width at half maximum in X-ray rocking curve of 90 seconds or below in the semiconductor light-emitting element according to the present invention, the underlayer can function as a buffer layer sufficiently and the crystallinity of the island-shaped single crystal portions constituting the light-emitting layer can be developed due to the high crystallinity of the underlayer.
In the semiconductor light-emitting element of the present invention, the in-plane lattice constant of the fifth semi-conducting nitride material constituting the island-shaped single crystal portion is set to be larger than the in-plane lattice constant of the third semi-conducting nitride material constituting the first cladding layer. In this case, compression stress is affected on the fifth semi-conducting nitride material, which results in being shaped in dot. That is, the island-shaped single crystal portions are formed on the compressive stress.
Therefore, by adjusting the sizes of the island-shaped single crystal portions appropriately, a given wavelength light is emitted from each of the single crystal portions, and as a result, an any color light or a white light is generated and emitted from the light-emitting layer including the single crystal portions at a luminous efficiency enough to be practically used.
If at least one of the forming temperature, the (V raw material/III raw material) ratio, the forming pressure is controlled in forming the island-shaped single crystal portions by a MOCVD method, the sizes of the single crystal portions can be easily adjusted.
The bandgap arrangement in the third semi-conducting nitride material constituting the first cladding layer, the fourth semi-conducting nitride material constituting the base layer and the fifth semi-conducting nitride material constituting the island-shaped single crystal portions is required to confine the island-shaped single crystal portions energetically and emit a given wavelength light from each of the single crystal portions.